Competition between preslip and deviatoric stress modulates precursors for laboratory earthquakes

Variations in elastic wave velocity and amplitude prior to failure have been documented in laboratory experiments as well as in a limited number of crustal earthquakes. These variations have generally been attributed to fault zone healing, changes in crack density, or pore fluid effects modulated dilatation or fault slip. However, the relationships between amplitude and velocity variations during the seismic cycle, and the underlying mechanisms of precursors to failure remain poorly understood. Here, we perform frictional shear experiments and measure the evolution of elastic wave velocity and amplitude throughout the laboratory seismic cycle. We find that elastic amplitudes and velocities undergo clear preseismic variations prior to fault failure. While preseismic amplitude reduction occurs early in the interseismic period, wave speed reduces later, just prior to failure. We perform a complementary set of stress oscillation experiments to quantify the response of seismic amplitudes and velocities to variations in the stress tensor. Taken together, our results indicate that preseismic amplitude variations are primarily controlled by fault slip rate and acceleration. On the other hand, elastic velocity responds to a combination of fault preslip which reduces seismic wavespeed and increasing stress in the wallrock, which increases wavespeed. Our data show that precursory changes in seismic wave speed may be more common than previously thought because they are masked by changes in wallrock stress. These results underscore the importance of continuous and long-term time-lapse monitoring of crustal faults for seismic hazard assessment and potential precursors to failure.